BACKGROUND: Polymyositis (PM) and dermatomyositis (DM) are associated with high risk of nasopharyngeal carcinoma (NPC) in Asian countries. Epstein-Barr virus (EBV) might induce autoimmunity and malignancies in susceptible individuals.OBJECTIVES: To investigate the association of EBV with PM/DM and NPC in PM/DM patients.STUDY DESIGN: Serum levels of anti-EBV viral capsid antigens (VCA) and anti-EBV-coded nuclear antigens-1 (EBNA-1) antibodies were measured by ELISA, and EBV DNA loads were determined using real-time PCR for 98 PM/DM patients, 94 systemic lupus erythematosus (SLE) patients and 370 healthy controls (HC). Anti-transfer-RNA synthetase antibodies (ASA) were determined by radioimmunoprecipitation for PM/DM patients.RESULTS: Thirteen (13.3%) of PM/DM patients vs. none of SLE patients had detectable NPC. ASA were detectable in 31.7% of PM/DM without malignancy, while lack of ASA in any PM/DM patient with NPC. IgA anti-EBNA-1 were detectable in 30.6% of PM/DM patients and 31.9% of SLE patients, but only in 4.1% of HC (odds ratio [OR] 10.44 and 11.12 respectively, both p<0.001). Significantly higher positivity for IgA anti-EBNA-1 were observed in PM/DM with NPC than in those without malignancy (OR 44.7, p<0.01). Significantly higher positivity for EBV genome were observed in PM/DM with NPC than in those without malignancy (OR 43.9, p<0.01), in SLE patients (OR 13.2, p<0.05) and in HC (OR 99.4, p<0.001). EBV DNA loads were significantly higher in PM/DM with NPC compared with those without malignancy and HC.CONCLUSIONS: Our results showed a positive association of EBV with PM/DM and NPC. PM/DM patients who have IgA anti-EBNA-1 or increased EBV DNA loads should be highly suspected to have occult NPC.